Distribution transformers are usually mounted in a protection casing which is provided with oil to act as a thermal carrier whereby heat generated by the windings is dissipated through the casing in contact with the oil. The use of oil and casing has proven to be problematic and also increases the physical size of the distribution transformer, making them difficult to directly bury them in the ground. Attempts have been made to bury distribution transformers in the ground as well as their cables which service the homes whereby to eliminate pole-mounted transformers or pad-mounted transformers. However, these buried transformers still required cooling when positioned underground and therefore the transformers were mounted in housings and the oil in the transformer or else the housing was cooled by a system of cooling pipes, also buried in the ground. An example of such distribution transformers construction is disclosed in U.S. Pat. Nos. 3,212,563 and 4,009,418.
With the advent of solid or dry transformers the size of the transformer has been greatly reduced but the cooling of such transformer is usually affected by heat exchange with air or a liquid circulated within the transformer structure or solid heat sinks in contact with ambient air. Some of the difficulties encountered with such solid or dry transformers are that heat dissipation through a solid dielectric material has proven to be poor and the result of thermal build-up can create hot spots or high thermal gradients which can crack the solid dielectric material. This can lead to serious breakdowns which require the replacement of the transformer. Also, most of these transformers require a grounded enclosure to remove any electrical shock hazard. Such enclosures are typically metal cages having dimensions much larger than the transformer itself, thus making installation space requirements difficult.
Recently, and with reference to U.S. Pat. No. 5,656,984, improved solid insulating transformers have been developed but these also require the casting of the entire transformer into a surrounding shell or housing. Such are usually mounted in very large manholes whereby to accommodate the cooling equipment which circulates the cooling liquid through the inner coils and core of the transformer to extract heat. Accordingly, such installations are fairly bulky and expensive. Recently, distribution transformers have also been developed wherein the transformer is fully protected by fiberglass and vinylester resin or else the transformer is casted in concrete. Such transformers are usually mounted in manholes and require some form of cooling medium.
U.S. Pat. No. 4,349,801 describes a single-phase transformer wherein the windings are cast in resin and wherein the wound cores are also protected from corrosion and mechanical damage in a suitable encapsulation of shock-proof construction whereby to permit the sealed transformer to be buried in the soil. Waterproof connector sockets are provided to connect underground cable thereto. FIG. 3 of the patent illustrates such a buried-type construction. These transformers are costly to fabricate, are bulky and develop the problems associated with overheating.